A large number of phosphors, including organic electroluminescent (EL) materials and inorganic EL materials, have hitherto been put to practical use. However, these phosphors have their crystallinity lowered mainly due to oxidation and humidification, and deterioration of the fluorescence properties with a lapse of time is noticeable. Industrial products making use of phosphors having such short lifetimes are well adapted to the mass consumption society where throwaway is considered as a virtue, but such industrial products cause a serious problem from the viewpoint of resource saving. Furthermore, in order to apply phosphors to illuminations, light sources or displays, it is imperative to realize low voltage driving from the viewpoint of energy saving. On the other hand, although practicalization has not yet been achieved, it is known that oxide polycrystals achieve a good balance between fluorescence properties and robust chemical stability. Furthermore, it is becoming obvious that when an oxide phosphor is incorporated into an EL element as a high quality thin film, the EL element can be expected to be driven at a low voltage such as about 10 V, unlike the case of other inorganic EL materials. Furthermore, area enlargement is necessary in general illumination and display applications.
Therefore, in order to increase the national economic and social sustainability by means of energy saving and resource saving, it is an urgent task to develop a technology which enables production of a high quality oxide phosphor thin film having fluorescence properties that can withstand practicalization, in a large area. A polycrystalline thin film formed of aggregates of three-dimensionally random crystals exhibit fluorescence properties, but since the intensity is very weak, it is necessary to obtain high intensity fluorescence by growing an oriented film with high crystallinity. Furthermore, because oxide phosphors have high transparency, the oxide phosphors are expected to be used as displays utilizing windows. However, in order to realize the expectations, it is needed to produce an oxide phosphor thin film on a transparent and inexpensive base material.
Non-Patent Document 1 discloses that a Langmuir-Blodgett film (LB film) of nanosheets composed of Ca2Nb3O10 is obtained on a glass substrate. Furthermore, Non-Patent Document 2 discloses that a crystalline thin film of SrTiO3 is obtained on a glass substrate by making an LB film of nanosheets composed of Ca2Nb3O10 into a seed layer. Furthermore, Non-Patent Document 3 discloses the red fluorescence properties of a polycrystalline layered perovskite Srn+1TiO3n+1 system. Non-Patent Document 4 discloses blue white fluorescence caused by oxygen deficiency in connection with SrTiO3 monocrystal and thin film. Non-Patent Document 5 discloses that red fluorescence properties are obtained by substituting polycrystal SrTiO3 with Pr atoms. Furthermore, Non-Patent Document 6 discloses red fluorescence properties of polycrystal Pr atom-substituted (CaSrBa)TiO3. Non-Patent Documents 7 and 8 disclose that an LB film of nanosheets composed of titanium dioxide and a cationic surfactant is obtained on a solid substrate.
Furthermore, Patent Document 1 discloses a method for producing a composite oxide phosphor thin film produced by substituting an inorganic parent material such as yttrium aluminate with metal ions. Patent Document 2 discloses fluorescence properties of a polycrystal Sn perovskite oxide system. Furthermore, Patent Document 3 discloses that red fluorescence is obtained with an epitaxial thin film of SrTiO3 substituted with Pr and Al atoms. Patent Document 4 discloses an EL element utilizing an oxide phosphor epitaxial thin film as a light emitting layer. Furthermore, Patent Document 5 discloses that a crystalline thin film of SrTiO3 is obtained on a glass substrate by making an LB film of nanosheets composed of Ca2Nb3O10 as a seed layer.